Liquid developer for electrophotography

ABSTRACT

THE INVENTION RELATES TO A DEVELOPER FOR ELECTROPHOTOGRAPHY OF THE TYPE COMPRISING A CARRIER LIQUID AND PIGMENT PARTICLES DISPERSED IN SAID CARRIER, THE INVENTION RESIDING IN THE CHEMICAL GRAFTING OF AN ORGANIC MATERIAL HAVING UNSATURATED DOUBLE BONDS TO THE PIGMENT TO CONTROL THE ELECTROKINETIC POTENTIAL OF THE SURFACE OF THE PIGMENT.

Un1ted States Patent US. Cl. 25262.1 2 Claims ABSTRACT OF THE DISCLOSURE The invention relates to a developer for electrophotography of the type comprising a carrier liquid and pigment particles dispersed in said carrier, the invention residing in the chemical grafting of an organic material having unsaturated double bonds to the pigment to control the electrokinetic potential of the surface of the pigment.

DETAILED DESCRIPTION OF THE INVENTION This invention relates to a liquid developer of improved stability for use in electrophotography. The improved developer contains pigments dispersed in a carrier liquid, the surface of said pigment being, in particular, chemically bonded with materials that control the electrokinetic potential of said pigments.

A conventional method of making liquid developer for electrophotography is to uniformly mix the pigments with resin, oil or fat by means of a heat roller, refrigerating and solidifying thereafter the resultant mixture, and grinding this mixture into a fine powder. The powdered compound is then filtered and then dispersed in the carrier liquid. Another method known in the art is similar to one employed in the manufacture of printing ink, in which resin, oil or fat and dyestuffs are thoroughly mixed in a ball mill, for example, into a liquid mixture to be dispersed in the carrier liquid. In both methods, however, the resin, oil or fat and dyestuffs are bound to drop off, when dispersed in the carrier liquid, from the surface of the pigment in the course of time, lowering the electrokinetic potential of the pigment and causing coagulation thereof, which in turn, deteriorates the quality of the developed image.

In general, the liquid developing process in electrophotography can be explained by electrophoresis of pigments dispersed in the carrier liquid; that is, the latent image to be obtained through corona charging and exposure of the sensitive layer, consisting essentially of zinc oxide, insulating resin and sensitizer and coated on the supporting member paper, is created by the difference of surface potential between the image area and the nonimage area. It is further presumed that there are lines of electric force which direct themselves from a low potential area towards a high potential area.

In the case of liquid developing process, therefore, depending on whether the dispersed pigments are positively or negatively charged, corresponding electrophoresis occurs either in the opposite direction to said lines of electric force, or in the same direction, and the latent image becomes visible by the electrodeposit of the pigments.

In this case the larger the electric charge, or the electrokinetic potential of the dispersed pigments, the greater becomes the velocity of electrophoresis, and therefore higher will be the image contrast in a given period of time. It follows that the dispersed pigments in the carrier liquid should desirably be of a high electrokinetic potential and of an unchangeable characteristic with the lapse of time.

For materials which would control the electrokinetic potential of the pigments, a combination of more than one of the resins such as alkyd resin, styrol resin, acrylic resin, oil and fat, such as linseed oil, paulownia oil, soya bean oil, or varnish, dyestuffs such as alkaline blue or spirit nigrosine and other polarity control agents such as asphalt, is conceivable, but the best feasible combination is obtainable only by checking each of the conceivable combinations, for one combination selected at random does not necessarily show an excellent electrokinetic potential over others.

On the other hand, if the dielectric constant of the carrier liquid were high, the lines of electric force becomes coarse, which, in turn causes, by the electrophoresic velocity of the pigments, deterioration of the image density to be obtained in a given period of time.

Further, if the resistance of the carrier liquid were lower, the electric charge, which is to form the latent image, is dispersed in the carrier liquid, which, in turn, also renders the lines of electric force coarse, causing the visible image to show poor contrast.

Carrier liquids of strong activity such as toluene, xylene, cyclohexane, perchloroethylene, trichloro-monofiuoromethane or trichloro-tri-fluoroethane, which dissolve away the sensitive layer, also lower the image density as they deprive the latent image of the electric charge thereon. It is for this reason that solvents having dielectric constants of no more than 3 and specific resistance of more than 10 9- cm., such as hexane, heptane, mineral turpentine, octane and cyclohexane, which are saturated hydrocarbons or hexyne and hexine, which are chain-type unsaturated hydrocarbons or isoparaflinic-hydrocarbons or even mixtures of the aforesaid, namely, ligroin or kerosene, are widely used as the carrier liquid.

However, although the carrier liquid so selected is generally chemically inactive, the pigment to be dispersed in the above-mentioned carrier liquid, on the other hand, carries on its surface, either by mechanical coating or chemical absorption, one or more than one of either of the resin, oil or fat and dyestuffs. And with the conventional pigments, the materials coated or absorbed on the surface of the pigment are gradually removed therefrom and carried into the carrier liquid in a period of time, thereby lowering the electrokinetic potential of the pigment, deteriorating the image density and causing elec trochemical colloidal coagulation of the pigment. The primary objective of this invention, therefore, is to provide a stable liquid developer for electrophotography by dispersing in the carrier liquid the pigment whose surface will be chemically bonded with more than one of either of resin, oil or fat and dye-stuffs, which control the electrokinetic potential of said pigment.

The invention is characterized in that materials which show excellent polarity control characteristics in the carrier liquid, such as unsaturated oil or fat, vinyl-monomer, linseed oil, or varnish having unsaturated double bond, are graft polymerized with the pigment by means of such radical initiators as peroxide materials,,like benzoyl peroxide, azobisphenol, or a,o!-azobisisobutyronitrile, with the addition, if necessary, of a polarity control agent, such as a dyestuff or other material which will be mixed and stirred with said pigment and dispersed in the carrier liquid so as to ensure higher stability of the developer in a period of time.

While it is not clear whether the graft polymerization of the unsaturated double bond materials on to the surface of the pigment is propagated by the action of a radical initiator upon such double bond as benzenoid ring of the pigment, or by the radical originally contained in the pigment being rendered as an active center, it is evident from the experiments hereinafter described that the p gment thus processed is chemically bonded by resin and other materials of unsaturated basis.

According to a specific example of this invention, the pigment, after being processed by lauryl acrylate together with peroxide, is dispersed in a liquid carrier such as ligroin. The electrokinetic potential of the dispersed colloidal graft pigment will be measured. Then, the pigment are taken out, by settling them with a centrifugal separator, to be further dispersed in a new liquid carrier. Measuring the electrokinetic potential again at this stage, will indicate little decrease from the first measurement. A small decrease in the electrokinetic potential of the pigments is observed when the above mentioned operation is repeated. Coagulation does not occur even if the pigments were left dispersed in the liquid carrier.

With the pigment absorbing materials on its surface mechanically or chemically by means of conventional methods, repetition of the aforesaid operation entails separation of the absorbed materials from the pigments, which substantially lowers the electrokinetic potential of the pigments to the extent that the developer becomes hardly usable for practical purposes.

When carbon black is used as a pigment and styrol monomer is graft polymerized thereon in the presence of a radical initiator, the electrokinetic potential is ll'lSllfilcient to render the compound usable as a developer for electrophotography and therefore requires the addition of a certain amount of metallic soap or dyestutf to make the compound usable. It is conceivable that with the developer of this kind, a dyestuff is inclined to drop off in the carrier liquid, but the polarity of the dispersed pigment is evidently controllable by the interaction of the added material. This is because the pigment which is graft polymerized and chemically bonded by styrol shows a higher stability as compared with that which is not so processed.

A saturated or unsaturated aliphatic hydrocarbon, which is primarily used as carrier liquid for the developer, is generally a poor solvent for resins and therefore, when styrol is used as monomer, the greater the extent of graft polymerization, the more difficult will it become for the resin to remain in the carrier liquid; one objectionable result is a noticeable settling.

For example, while the styrol monomer mixes well with n-hexane, the polystyrol polymerized to about 500 hardly dissolves in the n-hexane. It follows, therefore, that carbon black is desirably graft polymerized at a low polymerization degree i.e. about to 100. A method known in the art to obtain this low polymer is the use of a telemerization reaction, a method used in high molecular synthetic chemistry. Still another effective alternative is to use the monomer of acrylic acid derivatives etherified by a higher alcohol containing more than 8 carbons which is soluble in the saturated or unsaturated hydrocarbon.

The present invention is further illustrated in the following examples:

Example 1 After freezing and vacuum sealing in a polymerization tube containing a mixture of 0.5 gm. of carbon black (U.S.A. Cabot Corp. Elftex-S) dried for 3 hours at 105 C. in cc. of dehydrated carbon tetrachloride, 5 gm. of alkyd resin modified with soya bean oil (Dainihon Inki Chemical Industries, I-l352) and 0.1 g. of benzoyl peroxide, is allowed to react in the tube at 80 C. for 5 hours. Then, the tube is opened, the graft polymerized carbon black is filtered, the carbon black is washed thoroughly with petroleum ether and dried at low pressure. 200 cc. of mineral turpentine is added to 0.1 gm. of the resultant graft compound, mixing and dispersing it thoroughly in a hand mill and then dispersing the result in 480 cc. of the ligroin to produce the liquid developer.

Inasmuch as the electrokinetic potential of the abovementioned liquid developer is positive, developing on the 4 sensitized electrostatic paper of the latent image made from a positive original ensures a black positive image of high contrast. The liquid developer is also stable for a long period of time.

Example 2 After freezing and deflating in the polymerization tube, the mixture of 0.5 gm. of the carbon black (U.S.A. Cabot Corp. Mogul-A) is dried at C. for 3 hours in 20 cc. of dehydrated chloroform, 4.5 gm. of refined lauryl methacrylate, 0.5 gm. of diethylaminoethylmethacrylate and 0.1 gm. of a,a-azobisisobutyronitrile, is allowed to react in the tube at 60 C. for 3 hours. Then the tube i opened, 0.5% of calcium naphthenate and 2 cc. of toluene are added to the resultant compound, such compound being mixed for 8 hours in a ball mill to make a paste. After dispersing the paste in 500 cc. of kerosene a d veloper is obtained which is stable for a long period and suitable to obtain a positive image from a positive original.

Example 3 0.5 gm. of aniline black and 5 gm. of dodecyl methacrylate are graft polymerized using benzoyl peroxide. 0.5% of cobalt naphthenate and truene are thereafter added, and 2 gm. of the resultant paste is dispersed in 300 cc. of isoparafiinic-hydrocarbon. A developer is obtained which is stable for electrophotography and suitable for obtaining a positive image from a positive original.

I claim:

1. A liquid developer for use in electrophotography consisting essentially of a finely divided developing agent dispersed in a carrier liquid of the class consisting of hexane, heptane, mineral turpentine, octane, cyclohexane, hexene, hexyne, and isoparafifin hydrocarbons, said developing agent consisting essentially of the reaction product, prepared by graft-polymerization of a pigment of the class consisting of carbon black and aniline black and an organic compound of the class consisting of an alkyd resin modified with soya bean oil, dodecyl methacrylate and a mixture of lauryl methacrylate and diethylaminoethylmethacrylate, the pigment being in effective proportion to the carrier liquid to form a proper liquid developer from which the pigment can be deposited by electrophoresis for image-forming action, said organic compound being in effective proportions to the pigment to increase the stabilization of the electrokinetic potential of said pigment substantially against deterioration in the presence of the carrier liquid.

2. A liquid developer as described in claim 1, wherein the proportion of the organic compound to the pigment by weight is approximately 1 to 10.

References Cited UNITED STATES PATENTS 2,751,369 6/1956 Grotenhuis 260-41 2,780,612 2/ 1957 Grotenhuis 2604l 3,043,708 7/1962 Watson et al 106307 2,891,911 6/1959 Mayer et a1. 252-621 3,337,288 8/1967 Horiguchi 260-22 3,344,098 9/ 1967 Horiguchi 26040 X OTHER REFERENCES Chemical Abstracts, vol. 54, col. 23360c. Chemical Abstracts, vol. 52, col. 19231d.

JOHN D. WELSH, Primary Examiner US. or. X.R.

117-37; 961; 106-307, 288; 26041 R, 33.6 UA 

